diff --git a/src/libboard_artiq/src/cxp_proto.rs b/src/libboard_artiq/src/cxp_proto.rs
index fb4eed9..7793e24 100644
--- a/src/libboard_artiq/src/cxp_proto.rs
+++ b/src/libboard_artiq/src/cxp_proto.rs
@@ -169,12 +169,12 @@ pub enum RXPacket {
 }
 
 impl RXPacket {
-    pub fn read_from(reader: &mut Cursor<&mut [u8]>, packet_type: u8) -> Result<Self, Error> {
-        match packet_type {
+    pub fn read_from(reader: &mut Cursor<&mut [u8]>) -> Result<Self, Error> {
+        match reader.read_4x_u8()? {
             0x03 => RXPacket::get_ctrl_packet(reader, false),
             0x06 => RXPacket::get_ctrl_packet(reader, true),
             0x07 => RXPacket::get_event_packet(reader),
-            _ => Err(Error::UnknownPacket(packet_type)),
+            ty => Err(Error::UnknownPacket(ty)),
         }
     }
 
@@ -192,7 +192,9 @@ impl RXPacket {
                 let mut data: [u8; DATA_MAXSIZE] = [0; DATA_MAXSIZE];
                 reader.read(&mut data[0..length as usize])?;
 
-                let checksum = get_cxp_crc(&reader.get_ref()[0..reader.position()]);
+                // Section 9.6.3 (CXP-001-2021)
+                // only bytes after the first 4 are used in calculating the checksum
+                let checksum = get_cxp_crc(&reader.get_ref()[4..reader.position()]);
                 if reader.read_u32()? != checksum {
                     return Err(Error::CorruptedPacket);
                 }
@@ -241,7 +243,7 @@ impl RXPacket {
         let mut ev: [u8; DATA_MAXSIZE] = [0; DATA_MAXSIZE];
         reader.read(&mut ev[0..ev_size as usize])?;
 
-        let checksum = get_cxp_crc(&reader.get_ref()[0..reader.position()]);
+        let checksum = get_cxp_crc(&reader.get_ref()[4..reader.position()]);
         if reader.read_u32()? != checksum {
             println!("crc error");
             return Err(Error::CorruptedPacket);